Container treatment machines are used above all in the beverages and medicine industry as also in the chemical industry and are known in the most different of forms as stretching-blowing machines for the manufacture of plastic containers or as machines for cleaning, filling, sealing, printing and labelling bottles, cans, tubes or other containers made out of a plastics or metallic material. They are made as a general rule as rotary machines consisting substantially of a fixed machine part and a machine part rotating about a vertical machine axis, the rotating machine part being rotationally connected to the fixed machine part by means of a roller bearing arranged between the two machine parts and consisting of an outer bearing ring, an inner bearing ring and a plurality of rolling elements rolling between said bearing rings. On the rotating machine part are generally arranged a turntable and a plurality of treating stations comprising functional elements that are adapted to the respective type of treatment and interact with the containers to be treated. Depending on the size and the purpose of use of the plant, these turntables can have a diameter of up to 7 m and a weight of up to 7 tons and are operated at rotational speeds of up to 6 min−1 and, despite the resulting high load, their rotational speed must be precisely controllable in order to enable, for example, a compensation for fluctuations of capacity.
According to WO 2008/022737 A1, such container treatment machines are driven conventionally through a servomotor that uses an appropriate step-down gear unit to move a pinion that engages a gearing on the rotating machine part and thus makes it turn.
A drawback of such a conventional drive, however, is that this drive does not operate without wear and the pinion-and-gearing combination must be intensively greased regularly in order to assure the desired durability. However, as a result, it is possible for the surroundings to be contaminated by dripping lubrication grease which, with a view to the applicable hygienic regulations, is a drawback particularly in beverage bottling plants. In addition, due to the pinion-and-gearing combination, acoustic noise and body-borne sound as also mechanical dissipation of energy are generated during rolling, and this contributes to an undesired high noise level and is detrimental to the energy efficiency of the plant. Moreover, it has been noted in conventional drive concepts that in the case of an emergency shut-down, the machines driven by these drives cannot be decelerated by the motors in case of an emergency shut-down due to the high moment of inertia of the rotationally mounted machine part because this would overload and destroy the gear unit and the pinion. This therefore necessitates the use of additional mechanical brakes thus disadvantageously increasing work and costs required by the machine.
To avoid these drawbacks WO 2008/022737 A1 therefore proposes to make the drive of a container treatment machine as a direct drive using a torque motor, wherein in place of the hitherto used gearing, a continuous crown with a plurality of magnets is provided on the rotating machine part and, on the fixed machine part is arranged at least one stator made up of a plurality of electric coils that covers only a fractional sector of the magnet crown, so that through an electromagnetic field generated by energising the stator, in interaction with the magnets, the rotating machine part can be set into a defined rotational motion. The size of the stator and the number of integrated electric coils are variable and depend on the size, the needed torque and the required rotational speed of the turntable. For achieving a maximum torque, it is recommended to arrange a plurality of such stators around the crown of magnets.
Such a direct drive no doubt has the advantage that it no longer comprises a mechanical connection between the fixed and the rotating part and therefore operates free of wear. It has however proved to be disadvantageous because the individual stators cover the magnets only partly, so that many angular regions between the segment-shaped stators are freely accessible and constitute a potential source of danger for the assemblers and operators because of the high forces of attraction of the magnets arranged unprotected on the rotating machine part. It is likewise a drawback that, for supporting the magnetic forces of attraction acting in radial direction between the stators and the magnets, the individual segment-shaped stators are fixed on the fixed machine part by fastening screws that are screwed into the stators because as a result of this the fixed machine part and the bearing ring of the rolling bearing connected to this can get deformed due to the load being applied only segment-wise so that the functioning and the durability of the rolling bearing is limited. Over and above, it is a further drawback that, in particular, in the case of very large turntable diameters that are operated with very high torques, the magnets generally fixed on correspondingly large rolling bearings between the fixed and the rotating machine part are required in very large piece numbers and, due to the exposed angular regions between the individual stators, also in very large dimensions so that, because of their very high world market price, they cause very high manufacturing costs for the drive.